Nickel superalloys are materials currently used for making components intended for aeronautical turbines, such as turbine discs. These materials are characterized by their capability of operating under strong stresses and under strong fatigue loads at high temperatures, beyond 650° C., which may attain 1,090° C. in the case of certain applications of aeronautical turbines. The search for high-performance materials capable of withstanding increasingly higher operating temperatures is related to the need of improving the thermodynamic yield of the turbines.
The components for aeronautical turbines in nickel-based superalloys (i.e. including at least 50% by weight of nickel, the remainder consisting of various alloy elements) are most conventionally obtained via a route for obtaining them, a so-called “ingot route” where the nickel-based superalloy is elaborated by melting and re-melting, and then cast and shaped as an ingot, before being hot-worked with thermomechanical and thermal treatment(s) in order to obtain the desired microstructure and final shape.
This ingot route is, however, not optimum for making parts having the aforementioned superior properties, because of a microstructure which is not sufficiently homogeneous after melting and re-melting the alloy. Indeed, a very homogeneous microstructure of the material before hot-working is required in order to be able to work the material with greater deformation levels and deformation rates, while avoiding the formation of clinics (i.e. surface cracks formed during cooling) during the thermomechanical treatment and the occurrence of structural defects in the material.
Already for a few years, the so-called “powder route” for obtaining parts (powder metallurgy) with which materials having a much more homogeneous structure may be obtained, has been developed for making high performance components in nickel-based superalloys, notably for applications to aeronautical turbines. This powder route notably includes the following steps:                preparation of a melt having the targeted composition for the superalloy;        atomization of the melt in order to obtain a powder;        sifting this powder in order to only retain particles thereof having the desired grain size;        introducing the powder into a container, which is closed and put under vacuum;        densification of the powder and of the container in order to obtain an ingot or a billet of suitable dimensions;        thermomechanical treatments (forging, for example) and optionally heat treatments of the ingot or of the billet in order to obtain a final part with dimensions and structures suitable for the targeted application.        
However the parts obtained via the powder route are difficult to work by thermomechanical treatment, notably because of the lack of ductility of the parts obtained after densification of the powder.
The lack of ductility of the parts obtained from powders in nickel-based superalloys is explained by the characteristics of the surfaces of the original particles, which will mark the structure of the material and subsist after compacting the powder. The surfaces of the original particles are also known under the name of PPBs (Prior Particle Boundaries). The particles of the initial powder have surfaces which promote the formation and grouping of insoluble precipitates, such as oxides, sulfides, nitrides, sulfonitrides, carbides and/or carbonitrides which will subsist after compacting the powder. This phenomenon is known as “decorations” around the particles of powders. During the operation for compacting the powder, the precipitates present at the PPBs form stable lattices, the disappearance of which is not possible with subsequent treatments.
A consequence of this phenomenon is to promote interparticulate breakages during future stresses on the part, and to make it very difficult to enlarge the grain very substantially beyond the size of the original particles. Conventionally it is impossible to enlarge the grain beyond three times the sizes of the original particles. This makes the billet obtained after compacting of the powder very difficult to be forged and makes it impossible to obtain certain high final mechanical characteristics, such as good creep resistance.
In document EP-A-0 438 338 a solution was proposed with which the detrimental effects of the precipitates or decorations at the PPBs may be attenuated for nickel superalloys of the type with structural hardening by precipitation of the gamma′ phase, such as notably the alloys known under the commercial names of ASTROLOY®, UDIMET 720® or N18®. This document specifies the typical compositions of the ASTROLOY® and N18® alloys. The typical composition of UDIMET 720® is:                15.5%≦Cr≦16.5%        14%≦Co≦15.5%        4.75%≦Ti≦5.25%        2.25%≦Al≦2.75%        2.75%≦Mo≦3.25%        1%≦W≦1.5%        0.025%≦Zr≦0.05%        0.01%≦C≦0.02%        0.01%≦B≦0.02%        Ni=the remainder        
This solution consists of carrying out pretreatment of the superalloy, before its densification, at a temperature below the solvus temperature or close to the solvus temperature, of the gamma′ phase of the alloy (1,195° C. for ASTROLOY®, and 1,180° C. for N18®). With this method it is possible to attenuate the detrimental effect of the PPBs for superalloys hardened by gamma′ phase precipitation, by precipitating the segregated elements inside the particles of powders and not at their surface. By this pretreatment, decoupled from densification strictly speaking, the grains may become larger beyond the size of the initial particles, which allows an improvement in the forgeability of the alloy.
However, it is found that this solution, although providing remarkable technological advantages for nickel-based alloys with structural hardening by simple precipitation of the gamma′ phase, cannot be applied to nickel-based superalloys for which structural hardening is obtained by double precipitation of a gamma′ phase and of a gamma″ phase or delta phase.
Indeed, a pretreatment carried out under the solvus temperature of the gamma′ phase or in the vicinity of this solvus temperature does not, in their case, allow suppression or attenuation of the detrimental effect of the PPBs and decorations at the PPBs.
Nickel-based superalloys hardened by double precipitation, because of their mechanical properties (mechanical strength, creep resistance and resistance to fatigue at high temperatures), would have a great benefit for aeronautical applications, notably for the components of turbines such as the discs or the vanes. It would therefore be very important to find an elaboration method, via the powder route, allowing the use of these superalloys for these applications, such as, for example, the known superalloy commercially designated as 725®, because of its mechanical properties and of its corrosion resistance.